I recently downloaded MegaMari, which is a megaman game with a touhou twist to it. That's basically taking two of the most mentioned games in this topic (barring IWBTG because that wasn't hard moreso stupid) and forging them together into solidified death.
I'm not even kidding. Every enemy gets attack patterns as complicated as you'd expect from a touhou-based platformer, the incredible lack of extra lives at the start is almost never enough to get to the boss with even the slightest modicum of chance.
Did I mention all boss patterns are based off of actual touhou spellcards?
Though I have to admit, Marisa has a very small hitbox, and with some luck you'll get the patterns you can dodge the best (every amount of health you deplete does fresh out the boss' array of bullet pattterns though) so it's not incredibly impossible. But still it has to be one of the hardest games I have ever laid eyes on.
Also, some special levels out of the Vip Mario series can qualify, but I only know them through raocow playthroughs, so I'm not the best judge for their exact difficulty.